This invention relates to magnetic disk drives, and more particularly, to a head arm assembly having both servo and read/write heads thereon.
Magnetic disk units are extensively used in computer systems. Large magnetic disk units generally have a magnetic read/write assembly which is moved radially from track to track across a stack of magnetic disks. Examples of such drives are the IBM 3350 supplied by International Business Machines Corporation and STC 8350, supplied by Storage Technology Corporation. Recently, many small disk drives have been developed for minicomputer systems. An example of this type of drive is the IBM System 32 and comparable magnetic disk drives made by other companies. These disk drives usually make use of a magnetic head disk technology which is referred to as "IBM/Winchester".
High density magnetic disk drives have positional information recorded on one of the disks. A servo head reads this positional information to generate a servo signal which indicates the position of the read/write heads with respect to the magnetic disks in the stack.
In the prior art, the actuator for the magnetic head assemblies drives a plurality of read/write head arm assemblies having data read/write heads thereon, and one servo arm assembly having a single head thereon to generate the servo signal. The servo tracks are generally recorded on one of the extreme surfaces of the disk pack, for example on the bottom surface of the bottom disk in the pack. One reason for this arrangement is that it minimizes the interference between the data signals and the servo signals. However, the use of a dedicated arm assembly with a single servo head in this manner introduces other problems. If the axis of the stack of magnetic disks is tilted, an error is introduced between the track position indicated by the servo signal and the actual track position on disks other than the servo disk. When the servo track is on one of the extreme surfaces of the stack, this error is maximized on the disk surface at the other extreme of the stack. It would be more desirable to have the servo track intermediate the extremes of the stack. In this way, the off track error caused by tilt would be shared by the read/write heads on both sides of the stack from the servo disk. Also, the thermal environment experienced by the servo head would be more representative of the data heads and thus reduce arm related thermal distortion response. However, this is not feasible with a dedicated servo arm assembly with a single servo head as in the prior art. The use of such a dedicated servo arm in the middle of the stack would waste disk surface. The surface opposite and adjacent to the servo surface would not have read/write heads engaging it. Alternatively, an extra read/write head arm assembly would have to be provided for this otherwise unused surface.
It is an object of the present invention to provide a head arm assembly which minimizes interference between data and servo signals, which is positioned between the extremes of the disk stack, and which has both servo and read/write heads thereon, so that reading and writing can be accomplished efficiently from all disk surfaces.